Premium coke is well known in the art and is a commercial grade of coke having acicular, and anisotropic microstructure.
Premium cokes are used in the production of electrode grade graphite which requires that the coke have certain qualities. For example, a graphite electrode to be used in the arc melting of steel or the like must possess a low value for the coefficient of thermal expansion (CTE), particularly in the longitudinal direction, because of the severe thermal shocks which occur in such processes. The premium coke used for producing the graphite electrode must be capable of imparting a low CTE to the electrode.
In the process for producing a graphite electrode, a carbon body is formed from a coke and the carbon body is heated to between about 2000.degree. C. and about 3000.degree. C. to convert the carbon in the coke to a graphite crystalline form and to volatilize impurities. When a carbon body made from a coke is heated to temperature in the range of from about 1000.degree. C. to about 2000.degree. C., various sulfur-containing compounds present in the coke decompose, which often results in a rapid expansion and possible breakage of the carbon body. This phenomenon is termed "puffing". It is desirable to use a precursor containing a low amount of sulfur for producing the premium coke in order to minimize or preferably eliminate problems due to "puffing".
Typically, commercially produced premium cokes are made from aromatic, slowly reacting hydrocarbon feedstocks of low sulfur content, such as decant oils from catalytic cracking and tars obtained from the thermal cracking of decant oils and gas oils.
The presently used hydrocarbon feedstocks are satisfactory, but it would be desirable to use pyrolysis tars as feedstocks for producing premium cokes, because pyrolysis tars are relatively inexpensive mixtures of aromatic compounds and most of these tars have a low sulfur content. Generally, large amounts of pyrolysis tars are made as heavy by-products in the steam cracking of petroleum feedstocks to produce monomers, in particular ethylene, for the plastics industry.
Because of the high aromatic content and the low sulfur content, pyrolysis tars would seem to be suitable feedstocks for the formation of premium coke, but in fact they are generally unsuitable. Most pyrolysis tars are highly reactive, which causes problems in the delayed coking process, which is the process generally used to produce premium cokes. In this process the pyrolysis tars have a tendency to convert to coke in the coils of the delayed coker furnace under typical operating conditions. This results in clogging of the furnace, short operating periods, and excessive down time to clean the furnace coils. Another disadvantage is that cokes produced from pyrolysis tars are generally not premium cokes, that is they impart an undesirably high longitudinal CTE to graphite electrodes made therefrom. For these reasons, most pyrolysis tars are unsuitable for the production or premium coke.
H. O. Folkins in U.S. Pat. No. 3,817,753 discloses a method for upgrading pyrolysis tars by treating the pyrolysis tars with hydrogen in the presence of a conventional hydrodesulfurization catalyst. The catalysts are described as having a hydrogenation component on an inert carrier. Pyrolysis tars can be upgraded to some extent by the treatment with hydrogen in a Folkins process, using only a hydrogenation catalyst. However there is an undesirably high consumption of costly hydrogen and large losses in the final yield of coke. Furthermore, as shown in his Table 2, the CTE values for the cokes produced by the pyrolysis tars treated by the Folkins process (1.58.times.10.sup.-6 /.degree.C. and above) are unacceptably high for premium coke, which has a CTE below about 0.55.times.10.sup.-6 /.degree.C.
Hayashi, et al. in U.S. Pat. No. 4,312,742 discloses the treatment of various feedstocks, including pyrolysis tars, with hydrogen in the absence of a catalyst under gradual heating up to 350.degree. C. to 400.degree. C. They show the production of a final coke product with a marginally acceptable CTE (0.60.times.10.sup.-6 /.degree.C.). There is no disclosure of the CTE for the starting material or that the pyrolysis tar was upgraded by lowering of the CTE for the coke made from the tar. Furthermore, the process disclosed by Hayashi, et al. involves a gradual heating which would generally be commercially unacceptable because of the process time involved.
It is, therefore, an object of the invention to provide a method for upgrading pyrolysis tars such that they are suitable for the making of premium coke.
It is also an object of the invention to provide a method for the upgrading of pyrolysis tars with a low or negative consumption of hydrogen.
It is also an object of the invention to provide a method for the upgrading of pyrolysis tars without a high loss of the yield of coke.
It is also an object of the invention to provide a method for producing premium coke that imparts a low CTE to a graphite electrode.